tatting
(Not strictly a British term, but in the interests of clarity it's being
included here.) A kind of hand-made knotted lace, used particularly for
edging or trimming
(NSOED) -
the kind of lace on Ron's first set of
dress robes might have been tatting, for instance.

toerag
A worthless, despicable person.
This comes from an older sense referring specifically to vagrants, which
in turn comes from, well, a person who uses a rag wrapped around the foot
instead of a sock.
(NSOED).

U.S. readers may have already encountered this one in Douglas Adams'
The Long Dark Tea-Time of the Soul
as the name of one of the characters (Odin's nasty little sidekick).

"Imagine wasting your time and energy persecuting
merpeople when there are little
toerags like Kreacher
on the loose -"
- Sirius, speaking of
Umbridge's record
on part-humans
(OP14)

tosh
Nonsense. "The slang tosh is a blend of the words 'trash'
and 'bosh.' Bosh comes from the Turkish word for empty. Tosh
means 'nonsense' or 'empty, worthless talk.'"
(contributed by Sebastian Chen)

...the soles of his trainers were peeling away from the uppers
- Harry's scruffy appearance
(OP1)

treacle (as in treacle pudding or treacle tart)
Treacle is a by-product of the sugar refining process and can vary in
grade from very light Golden Syrup to Black Treacle (rather like
molasses). Treacle Pudding is a plain steamed suet pudding which has
warmed treacle poured over it. Treacle tart is a flat pastry case
filled with treacle mixed with breadcrumbs and baked. In both dishes,
Golden Syrup is usually used, as real treacle is quite strong in
taste.

Blocks of ice-cream in every flavour you could think of, apple pies,
treacle tarts...
(PS7)

He felt it was a better use of his time to eat his way steadily
through his steak and kidney pie,
then a large plateful of his favourite treacle tart.
- Harry, tuning out
Ron and
Hermione's bickering
(GF11)

treat
Very good or very well. If something "looks a treat"
it looks great, i.e., a treat to the eyes. "A treat"
is a common phrase in some dialects, and can be used in other contexts,
e.g., "I've polished this goblet and it's come up a treat".

"Tell yeh what, come with me an' see the
Great Hall,
looks a treat."
(PS12)

trifle
A layered dessert of sponge cake soaked in sherry, topped with chopped
fruit in jelly (jello), topped with custard, topped, in turn, with whipped
cream. The top of that may be decorated with angelica, glace cherries,
chocolate flakes or hundreds and thousands (tiny rainbow sugar candy pieces).

"I have only two words to say to you," he told them, his
deep voice echoing around the hall. "Tuck in."
- Albus Dumbledore
(GF12)

tuffet
A tuft of stringy things (such as blades of grass, threads, or
hairs) held together or growing together at a common base.
Note that this isn't the same sense of the word as in the
nursery rhyme "Little Miss Muffet / sat on a tuffet";
the word can also mean a little hill or mound.

They didn't have breath to spare for talking as they began to climb
Stoatshead Hill,
stumbling occasionally in hidden rabbit holes, slipping on thick black
tuffets of grass.
(GF6)

turf out
U.S.: throw out.

...casually turfing a first-year out
of one of the good armchairs by the fire so that he could sit
down.
(HBP12)

He turfed out half the contents of his trunk before he found
it hiding beneath the rolled-up socks in which he was still
keeping his bottle of lucky potion,
Felix Felicis
(HBP18)

twiddle
Twiddle means to play around with, perhaps uncertainly, and is also
a synonym for fiddle or fiddly;
e.g.twiddly bits
can be read as fiddly bits.
Twiddly usually refers to something
small - often a control of some sort that can be turned or rotated;
e.g.,
a volume control can be twiddled, a light switch cannot (unless it is a
rotary dimmer).
Twiddled is usually used in a technical sense,
e.g.,
"He didn't know why the radio wouldn't work, so he twiddled the knobs a
bit." (contributed by Pat Gilliland)

"Listeners, that brings us to the end of another
Potterwatch.
We don't know when it will be possible for us to broadcast again, but you
can be sure we shall be back. Keep twiddling those dials..."
(DH22)

twill
A woven fabric, sometimes used in making clothing, with a surface of
diagonal parallel ridges, produced by passing the weft threads over
one and under two or more threads of the warp (instead of over and
under in regular succession, which is how a lot of fabric is woven).
(The warp threads on a weaving loom are those that are strung by the
moving part of the loom; the weft are the threads at right angles to
the warp.) The word "twill" can also refer to the
characteristic appearance of such fabric and the process of making it
(NSOED).